4 Laminae, Laminasets, Beds, and Bedsets

Abstract

ABSTRACT This chapter discusses the smaller scales of the stratal hierarchy—from lamina to bedset. In mudstone, these typically range from less than a millimeter to hundreds of millimeters in thickness. This is the scale of strata that records individual depositional events and environmental changes in bottom energy, biogenic production rates, and redox conditions. Recognition and description of this scale of strata enables recognition of repeated patterns and associations of rock-property variation (facies), their distribution in three dimensions, and interpretation of the proximate causes of variation (to enable prediction away from sample control). In this chapter, we define essential stratal elements from laminae (the smallest units) to bedsets (larger units) and provide key recognition criteria and examples of these elements in cores, outcrops, and thin sections. Bohacs et al. (2022, Chapter 5 this Memoir) discusses how beds and bedsets stack into parasequences and how to synthesize this next larger scale of strata into an overall picture of a depositional environment (in terms of sediment supply; dominant erosional, transport, and depositional processes; and oceanographic conditions)—and make the tie with the well-log response. Bedding is a key characteristic of sedimentary rocks (as introduced in Lazar et al., 2022a, Chapter 2 this Memoir); it records variations in sediment input and accumulation, as well as benthic energy and the effects of sediment disruption by organisms. Our approach to describing bedding builds upon Campbell’s (1967) work, which emphasizes the genesis of the bed’s characteristics that reveal depositional conditions and history. Bedding is described by two sets of essential attributes: (1) the shape and geometry of bed bounding surfaces and (2) the continuity, shape, and geometry of laminae between the bounding surfaces (in this context, “shape” denotes the spatial configuration of a lamina or surface, whereas “geometry” signifies the spatial arrangement of bedding elements with respect to the surrounding bedding elements, i.e., parallel or nonparallel).